Chad Taylor sent a letter to the Kansas secretary of state withdrawing from the race, which also has a viable independent candidate, Olathe businessman Greg Orman, as well as a Libertarian candidate, Randall Batson, of Wichita.

"Effective today, my campaign is terminated," he said. Originally, the statement said "suspended," but the word was crossed out and replaced with a handwritten "terminated."

Kansas Democratic Party officials weren't saying how they planned to respond to Taylor's withdrawal.

Secretary of State Kris Kobach, a Republican and former law professor, said his initial reading of state election laws is that they require the party to pick a new nominee. But he said he'll consult with his legal staff Thursday.

Roberts' executive campaign manager Leroy Towns called the move a "corrupt bargain" between Democratic leaders and Orman's campaign. Both the Taylor and Orman camps declined to comment beyond written statements.

Taylor is the district attorney in Shawnee County, home of the state capital of Topeka. He won the office in 2008 and was re-elected without opposition in 2012. Orman had positioned himself as a nonpartisan centrist — and as Roberts' most formidable opponent in the Nov. 4 general election.

Campaign finance records show Taylor raised about $163,000 in contributions from November through July, while Orman took in more than $670,000 after starting his campaign in May. Roberts raised about $3.4 million from the beginning of last year through July, but he had a tough primary race against tea-party challenger Milton Wolf.

Roberts remains favored to win the race in GOP-leaning Kansas, though he received just 48 percent of the vote in his primary race against Wolf and two lesser-known candidates. Republicans have won every U.S. Senate contest since 1932, and they enjoy a nearly 20 percentage-point advantage among the state's 1.74 million registered voters.

Roberts has since sought to unify Republicans by appealing to their frustrations with Democratic President Barack Obama and Democratic U.S. Senate Majority Leader Harry Reid.